Encounter With the Infinite

The Gateway Engine sits at the literal centre of every planet. Activate at least one surface chamber first (First Step Into a Larger World) or its forcefield stays up, then dig: straight down with a Drill Mod, or follow the cave systems. Gravity fades as you go - the last stretch is a gentle float into the glowing structure, and stepping inside pops this. You'll cross Delve Greedily and Deep on the way down.